Spectroscopy was one of the most important methods for trace gas detection. External cavity quantum cascade laser (EC-QCL) had become an important laser light source in trace detection system. Because the output of the laser was broadly tuning, narrowly line width, and mid-infrared band. For on-line detection of toxic and harmful atmospheric pollutants, Littrow outer cavity technology was used to realize the laser center-wavelength of 11.4μm broadly tuning 827.7 cm^-1~928.7cm^-1(10.7μm ~12.08μm) in the mid-infrared band, and the line width less than 1cm^-1. The narrowly line width and broadly tuning laser output in this band is the first reported in China. The laser solves the problem of simultaneous on-line detection of trace toxic and harmful gases such as trichloroethylene, phosgene and naphthalene.